The present invention relates to a spark plug which can be mounted into a plug hole formed in a cylinder head without need of screwing into the plug hole, and a structure for mounting the spark plug.
A spark plug 10 used as an ignition source to a fuel-air mixture in the internal combustion engine comprises a shaft-shaped central electrode 3, an insulator 2 for surrounding the central electrode 3, and a main metal shell 1 for holding the insulator 2, as shown in FIG. 1, for example. The main metal shell 1 has a flange portion 11 with a plug bearing surface 13 formed on the tip side (where a spark discharge gap g is formed) and a top end portion 12 extending from the plug bearing surface 13 to the top end in an axial direction O. On a top end face of this top end portion 12, an ground electrode 4 for forming a spark discharge gap g in cooperation with the central electrode 3 is connected to form a connecting portion (ground electrode connecting portion) 17. Further, an external thread 12a is formed around an outer circumference of this top end portion 12. Also, a caulking portion 16, a hexagonal portion 15 and a caulking groove portion 14 are formed on the rear end side of the flange portion 11, with the external thread 12a formed around the outer circumference of the top end portion 12.
More particularly, the insulator 2 comprises a head portion 2b formed with a corrugation 2c, a diameter portion 2e having the greatest diameter, a middle trunk portion 2g having a smaller diameter than the diameter portion 2e, and a leg length portion 2i located at the top-most end. Generally, an engagement face 2h, which is a connecting portion between the leg length portion 2i and the middle trunk portion 2g, is engaged with a main metal shell side engaging portion 18 (more particularly a face for engaging the insulator) protruded inwardly into an inner circumferential face of the main metal shell 1 via a packing 51. The engagement face 2h of this insulator 2 is intimately contacted with the main metal shell side engaging portion 18 via the packing 51 to retain air-tightness between the insulator 2 and the main metal shell 1, and makes a part of a main passage for diffusing a heat flowing into the central electrode 3 and the leg length portion 2i of the insulator 2, through the insulator 2, the packing 51, the metal shell side engaging portion 18, and the top end portion 12 (external thread 12a) to a cylinder head as will be described later.
In addition, this spark plug 10 is mounted on the cylinder head of the internal combustion engine, when in use. When the spark plug 10 is mounted on the cylinder head, the spark plug 10 is inserted into the plug hole formed in the cylinder head, and the external thread 12a formed at the top end portion 12 of the main metal shell 1 is screwed into an internal thread formed in the plug hole at a predetermined torque, with a plug wrench set on the hexagonal portion 15 of the main metal shell 1.
When the spark plug 10 is correctly rotated around a central axis of the plug hole having the internal thread therein, the spark plug 10 can be fastened into the plug hole without problem. However, if the spark plug 10 is rotated in a state of being slanted from the central axis of the plug hole, the external thread 12a is not correctly mated with the internal thread, resulting in a so-called gall state. Therefore, the external thread 12a or the internal thread may be damaged, and in extreme cases, a blow-by of a combustion gas from the combustion chamber is incurred so that the top end portion 12 of the main metal shell 1 or the central electrode 3 is heated to high temperatures, with the risk of possibly giving rise to a preignition or melting away the electrode.
In order to prevent the possible risk from generating, it can be conceived to construct a spark plug mounting structure as follow. The spark plug 10 (top end portion 12) may be inserted with a fine clearance into the plug hole, while the external thread 12a at the top end portion 12 of the main metal shell 1 and the internal thread in the plug hole are not formed. The spark plug 10 is then secured to the cylinder head by a plug fixture separately.
However, since there is a clearance between the top end portion 12 and an inner wall face of the plug hole with a constitution having no external thread 12 at the top end portion 12, the diffusion of the heat becomes difficult. Therefore, the temperature of the central electrode 3 is prone to rise, leading to the risk of giving rise to preignition or melting away the electrode.
This invention has been achieved in the light of the aforementioned problems, and it is an object of the invention to provide a spark plug and a spark plug mounting structure for mounting the spark plug into a plug hole formed in a cylinder head, in which the spark plug is easily mounted in the plug hole and can diffuse the heat excellently to have an improved heat resistance.
The above object can be achieved by, according to the present invention, a spark plug mounting structure comprising:
a spark plug including,
a central electrode extending in an axial direction thereof,
an insulator having a tip end portion which surrounds the outer circumferential face of the central electrode, and
a main metal shell holding the insulator, the main metal shell having a flange portion with a plug bearing surface at a tip end of the flange portion, and the main metal shell having a top end portion which extends in an axial direction thereof from the plug bearing surface to a tip end side of the main metal shell; and
a cylinder head having plug hole with a top end couterpart which receives the top end portion of the main metal shell,
wherein the top end portion of the main metal shell is formed into a substantially cylindrical shape having an outer circumferential face on which no external thread is provided, and the top end portion of the main metal shell and the top end counter part of the plug hole satisfies a following relation,
xcfx86Dxe2x88x92xcfx86dxe2x89xa60.15
xe2x80x83where xcfx86d (unit: mm) defines the outer diameter of the top end portion, and xcfx86D (unit: mm) denotes the inner diameter of the top end counterpart.
Also, according to the invention, there is provided a spark plug adapted to be mounted in a plug hole formed in a cylinder head, the spark plug comprising;
a central electrode extending in an axial direction thereof;
an insulator having a tip end portion which surrounds the outer circumferential face of the central electrode; and
a main metal shell holding the insulator, the main metal shell having a flange portion with a plug bearing surface at a tip end of the flange portion, and the main metal shell having a top end portion which extends in an axial direction thereof from the plug bearing surface to a tip end side of the main metal shell,
wherein the top end portion of the main metal shell is formed into a substantially cylindrical shape having an outer circumferential face on which no external thread is provided, and the outer diameter of the top end portion satisfying a following relation,
xcfx86Dxe2x88x92xcfx86dxe2x89xa60.15
xe2x80x83where xcfx86d (unit: mm) defines the outer diameter of the top end portion of the main metal shell, and xcfx86D (unit: mm) denotes the inner diameter of a top end counterpart of the plug hole where is in confront with the top end portion.
With such a constitution of the invention, the top end portion of the main metal shell constituting the spark plug is formed substantially cylindrical without having a plug attaching external thread on the outer circumference. Thereby, when mounted, the spark plug is only inserted with a fine clearance into the plug hole, without needs of the screwing operation employing a plug wrench. In particular, the DOHC engines lately developed have mostly the suction and exhaust valves of large area, and the plug hole is designed deep, but the spark plug can be mounted only inserting with a play into the plug hole of this engine. Accordingly, the spark plug can be easily mounted without fear of damaging the thread portion which is occurred for a conventional spark plug when the external thread of the main metal shell is not mated with the internal thread of the plug hole.
The plug hole may has a flange counterpart and a top end counterpart corresponding in the outer peripheral shape to the flange portion and the top end portion of the main metal shell, respectively.
By the way, since the top end portion of the main metal shell is shaped substantially cylindrical on the outer circumferential face, the heat that could be conventionally escaped from the external thread formed in the top end portion to the cylinder head (plug hole inner wall face) becomes difficult to be diffused. Namely, assuming that the outer diameter of the top end portion in the main metal shell is xcfx86d (unit: mm), and the hole diameter of the top end counterpart in the plug hole is xcfx86D (unit: mm), when the clearance amount (xcfx86Dxe2x88x92xcfx86d) is extremely small, the heat flowing into the main metal shell can be sufficiently diffused into the plug hole inner wall face. When this clearance amount (xcfx86Dxe2x88x92xcfx86d) is relatively large, the passage for diffusing the heat becomes insufficient.
Namely, there is a relatively large air layer (layer formed by the air) interposed in the clearance between the top end portion of the main metal shell and the plug hole inner wall face (top end counterpart of the plug hole). And when this air layer is exposed to the combustion gases at high temperatures, this air layer functions as an adiabatic layer to prevent the heat flowing into the top end portion of the main metal shell from being diffused to the plug hole inner wall face. Therefore, the temperatures of the insulator, the central electrode and the ground electrode are prone to rise, possibly giving rise to a preignition or melting away the electrode.
On the contrary, with the spark plug of the invention, it should be noted that the clearance amount (xcfx86Dxe2x88x92xcfx86d) is set to satisfy the relation xcfx86Dxe2x88x92xcfx86dxe2x89xa60.15 mm With the mounting structure having the clearance amount (xcfx86Dxe2x88x92xcfx86d) set to satisfy the above relation, the heat flowing from the insulator, the central electrode and the ground electrode into the main metal shell can be diffused through the top end portion of the main metal shell into the cylinder head rapidly. As a result, the central electrode, the insulator and the ground electrode are maintained at low temperatures, with the heat resistance being equivalent or superior to that of the spark plug having formed the external thread in the top end portion, whereby the spark plug and the mounting structure can be produced which have less risk of giving rise to preignition or melting away the electrode. The range of the clearance amount (xcfx86Dxe2x88x92xcfx86d) is preferably 0.05 mmxe2x89xa6xcfx86Dxe2x88x92xcfx86dxe2x89xa60.15 mm in the light of the easiness of inserting the spark plug into the cylinder head, and more preferably 0.05 mmxe2x89xa6xcfx86Dxe2x88x92xcfx86d 0.10 mm in view of the easiness of inserting the spark plug and the good heat resistance.
The reason why the heat resistance of the spark plug can be obtained excellently when the clearance amount (xcfx86Dxe2x88x92xcfx86d) is less than or equal to 0.15 mm is that the thickness of the air layer itself interposed is extremely reduced, there is less adiabatic effect with this air layer, and the heat from the top end portion can be effectively diffused via the air layer into the plug hole inner wall face. Also, another reason is that because the main metal shell is made of metal material, when the clearance amount (xcfx86Dxe2x88x92xcfx86d) is less than or equal to 0.15 mm, the main metal shell (top end portion) is exposed to the combustion gases at high temperatures to cause the top end portion to be thermally expanded, and allow the top end portion to come into contact with the plug hole inner wall face effectively. Consequently, it is conceived that the passage for diffusing the heat from the top end portion of the main metal shell to the plug hole inner wall face can be effectively secured.
By the way, the heat flowing into the central electrode or the insulator is passed by way of the metal shell side engaging portion (more particularly the face for engaging the insulator in the metal shell side engaging portion) that is protruded inwardly around the inner circumferential face of the main metal shell to engage the insulator, as its diffusion passage. And this heat is diffused from the metal shell side engaging portion through the top end portion into the cylinder head (plug hole inner face) and through the plug bearing surface of the flange portion into the cylinder head (plug hole inner wall face). From this, the plug bearing surface is one of the main passages for diffusing the heat flowing into the central electrode or the insulator, in addition to the top end portion of the main metal shell.
Thus, with the spark plug as described above, the distance L (unit: mm) measured axially toward the tip from a top end of the plug bearing surface to a base end of the shell side engaging portion should satisfy a relation xe2x88x926xe2x89xa6Lxe2x89xa66.
With such a constitution, the heat flowing from the insulator or the central electrode into the metal shell side engaging portion (more particularly face for engaging the insulator of the metal shell side engaging portion) can be passed into the top end portion, and the plug bearing surface easily, and the heat flowing into the insulator or the central electrode can be diffused efficiently from the plug bearing surface and the top end portion into the cylinder head (plug hole inner wall face). As a result, the central electrode and the insulator can be maintained at low temperatures, making it possible to provide the spark plug superior in the heat resistance and the mounting structure thereof. Note that the rang of the distance L may be preferably in the relation xe2x88x923xe2x89xa6Lxe2x89xa66 in respect of the excellent heat resistance.
Herein, the base end of the metal shell side engaging portion that is a reference for measuring the distance L indicates the edge closest to the base end side in the metal shell engaging portion, when the face for engaging the insulator is slanted with respect to the axis, or tapered. On the other hand, when the face for engaging the insulator is perpendicular to the axis, or flat, the axial position of the base end side edge coincides with the axial position of the face for engaging the insulator. Similarly, the top end of the plug bearing surface indicates the edge closest to the top end side, when the plug bearing surface is slanted with respect to the axis, or tapered. On the other hand, when the plug bearing surface is perpendicular to the axis, or flat, the axial position of the top end coincides with the axial position of the plug bearing surface.
Further, the spark plug according to the invention comprises one or more ground electrodes having one end connected to the main metal shell to form an ground electrode connecting portion, with the other end being opposed to the central electrode via a spark discharge gap, wherein at least one of a main portion and a top end portion of the main metal shell is made a fitting position restriction shape to be fitted at one or more positions with at least one of the main portion counterpart and the top end counterpart in the plug hole, when rotating the spark plug axially while inserting the spark plug into the plug hole, wherein the relation between one or more formation positions of the ground electrode connecting portion and at least one of the main portion and the top end portion of the main metal shell that is made the fitting position restriction shape is substantially equivalent with regard to the spark plugs with the same parts number.
With such a constitution, at least one of the flange portion and the top end portion of the main metal shell is made the fitting position restriction shape with respect to the plug hole (at least one of the flange counterpart and the top end counterpart of the plug hole), and the relation between one or more formation positions of the ground electrode connecting portion and the flange portion or the top end portion of the main metal shell that is made the fitting position restriction shape is substantially the same. In this way, if the spark plug is inserted and fitted into the plug hole, the ground electrode connecting portion is restricted to a specific position in the circumferential direction around the axial line of the plug hole itself. Namely, only by inserting the spark plug into the plug hole, the ground electrode of the spark plug can be set at the specific position within the combustion chamber easily and securely.
Usually, it is known that to keep the ground electrode from impeding the flow of a swirl arising at the compression stroke, there is the optimal positional relation between the swirl flow direction and the formation position of the ground electrode connecting portion. The positional relation between the swirl flow direction and the ground electrode connecting portion is taken into account beforehand to restrict the relation between the formation position of the ground electrode connecting portion and the flange portion or top end portion that is made the fitting position restriction shape, so that the relation between the swirl flow direction and the formation position of the ground electrode connecting portion can be held constant at any time for each cylinder or internal combustion engine. As a result, there is less dispersion in the ignitability for the cylinders or internal combustion engines, and there is excellent ignitability, so that the internal combustion engine can be driven at uniform air fuel ratio (A/F), and at lean air fuel ratio.
Herein, the fitting position restriction shape may be arbitrary, so far as the spark plug can be fitted at one or more positions into the plug hole, if inserted into the plug hole while being rotated axially, in disposing the flange portion or top end portion of the main metal shell corresponding to the flange counterpart or top end counterpart of the plug hole. More specifically, the key groove extending axially may be formed in the plug hole, and the key projection (streak or projection) fitted into the key groove provided in the top end portion, or conversely, the key projection may be provided in the plug hole and the key groove provided on the main metal shell side. Further, the flange portion of the main metal shell may be beveled and the plug hole adapted to this beveling. Additionally, the main portion and the flange counterpart of the plug hole may be shaped to be oval, elliptical or polygonal in cross section.
Further, the spark plug may further comprise a barrel portion integrated with the main metal shell of the spark plug to be unrotatable about the axis thereof, and an ignition coil portion, which is accommodated within the barrel portion and has a primary winding and a secondary winding, for applying a high voltage for ignition that is generated in the secondary winding to the spark plug, thereby constituting a coil integral-type spark plug.
Employing this coil integral-type spark plug, when mounted into the plug hole, the spark plug can be inserted with a play into the plug hole formed in the deep region such as the DOHC engine, and mounted only by securing a projection portion formed on the barrel portion with the cylinder head. Namely, the coil integral-type spark plug of the invention does not need the screwing operation of the main metal shell employing a plug wrench, and can be easily mounted. Accordingly, there is less fear of causing a galling state by rotating the spark plug notwithstanding the external thread groove of the main metal shell does not mesh with the internal thread groove of the plug hole.
In this invention, when the clearance amount (xcfx86Dxe2x88x92xcfx86d) between the bore diameter xcfx86D of the top end counterpart for the plug hole and the bore diameter xcfx86d of the top end portion for the main metal shell is as quite small as 0.15 mm or less, an operation of mounting the spark plug into the plug hole can be carried out easily and securely, employing the coil integral-type spark plug.
Further, employing the coil integral-type spark plug, the plug bearing surface of the flange portion can be stably brought into contact with the cylinder head directly or indirectly via the other member (specifically a gasket).